The present invention relates to an object detector employing an infrared pyroelectric device.
An object detector having an infrared sensor is generally used in a control device for an automatic door or in a burglar alarm system. Such an infrared object detector senses an object present in a specific detection area. FIG. 1 illustrates a conventional object detector disclosed in Japanese Patent laid-open Publication No. Heisei 4-324329. The conventional object detector has a sensor 100 for sensing an object and a signal processor 200 for signal-processing information of the sensed object.
Referring to FIG. 1, light receiving windows 110 and 120 are installed at frontal surfaces of both ends of the sensor 100, and hoods 130 and 140 are provided around the light receiving windows 110 and 120 to define the edges of light receiving views. Infrared lenses 150 and 160 and infrared sensor modules 170 and 180 connected to the signal processor 200 are disposed behind the light receiving windows 110 and 120, respectively.
Referring to FIG. 2, the infrared sensor module 170 includes a pyroelectric device 172 and a field effect transistor (FET) 173 in a case 171. Infrared light incident through the light receiving window 110 is detected by the pyroelectric device 172, which outputs a corresponding electrical signal. This signal is impedance-transformed by the FET 173 and then output to the signal processor 200 of FIG. 1.
Signals output from the infrared sensor modules 170 and 180 are amplified by amplifiers 211 and 212, respectively, of the signal processor 200, then sent to comparators 221 and 222. The outputs of the comparators 211 and 222 are processed by an AND gate 230. From the output of the AND gate 230 it can be determined whether an object exists in a detection area.
In the conventional infrared object detector as constituted above, a detection area D (see FIG. 1) in which an object can be detected is defined as where the light receiving views of the pair of light receiving windows 110 and 120 intersect each other. That is, when an object enters the detection area D, the periodic superimpositions of signals output from the infrared sensor modules 170 and 180 behind the respective light receiving windows 110 and 120 occur. Therefore, the absence or presence of an object in the detection area D can be determined.
However, if the object enter at a short or long distance from the sensor 100, the superimpositions of signals are not generated, thus making it difficult to determine the absence or presence of the object. Further, the presence of the object cannot be determined three-dimensionally because the detection area of the object is limited to two-dimensions. In addition, the sensor 100 having the sensor modules is too bulky, making utilization of the conventional infrared object detector difficult for small products such as a camcorder, and the output signal of the pyroelectric device varies greatly as ambient temperature changes, thereby lowering the reliability of the conventional infrared object detector used, for example, for optimum control of the output of an air conditioner.